It is known to use check valves in tank-venting arrangements. The disclosure utilizes the term "overpressure valve" for a check valve which opens in response to overpressure in the tank while the term "underpressure valve" is used for a check valve which opens in response to underpressure in the tank.
U.S. Pat. No. 4,872,439 discloses a tank-venting arrangement having the following components: a tank having a tank closure; an active charcoal filter; a first connecting unit between the tank and the active charcoal filter wherein an electrically controllable check valve is present; a second connecting unit between the tank and the active charcoal filter wherein an overpressure valve and an underpressure valve are connected in parallel; a tank-venting valve; a tank closure sensor which supplies a tank opening signal when the tank closure is opened; a switch unit which receives the tank opening signal and opens the check valve in response thereto; and, a control unit for controlling the tank-venting valve.
The valves in the connecting units between the tank and the active charcoal filter ensure that an overpressure builds up in the tank which has the desired consequence that the vapor of the fuel in the tank is reduced. Fuel vapor flows into the active charcoal filter where it is adsorbed only when the pressure in the tank increases above the pressure at which the underpressure valve opens. As soon as the control unit drives the tank-venting valve, the active charcoal filter is flushed with air with the adsorbed fuel being desorbed. The flushing air charged with fuel vapor is supplied to the intake air of the internal combustion engine on which the arrangement is provided.
The case of tanking is problematic in such a tank-venting arrangement with overpressure. The tank cap can fly off because of overpressure when the cap is opened. In order to prevent this condition, the known arrangement has the above-mentioned tank closure sensor which ensures that the check valve leading to the active charcoal filter is opened when the tank closure is opened. In this way, the overpressure in the tank decays before the tank cover is unscrewed.
Underpressures of various magnitude develop in the active charcoal filter in dependence upon the operating condition of the engine on which the tank-venting arrangement is operated. These underpressures produce effects which extend to the above-mentioned overpressure valve which causes this valve to be already partially open when the pressure in the tank lies only insignificantly above the ambient pressure. In this way, the actual purpose of the tank-venting arrangement designed for overpressure is lost, namely, to reduce the degree of vaporization of the fuel in the tank. The above-mentioned unwanted effect can be reduced in a way which ensures that the active charcoal filter is vented as well as possible; however, a very good venting can only be achieved with a new filter. The underpressure acting on the overpressure valve becomes evermore intense with increasing age of the filter.